The invention relates to a colour display tube comprising in an evacuated envelope an electron gun system of the "in-line" type for generating three electron beams situated with their axes in one plane. The axis of the central beam coincides with the tube axis, and the electron beams converge on a display screen which is provided on a wall of the envelope. In operation, the beams are deflected over the display screen in two mutually perpendicular directions by means of a first and a second deflection field. The direction of the first deflection field is parallel to the plane. The electron gun system comprises at its end curved field shapers for causing the rasters described on the display screen by the electron beams to coincide as much as possible each field shaper comprises at least two plates of ferromagnetic material situated substantially in the elongation of each other and separated by gaps. The plates are symmetrical with respect to the plane and the central beam axis. The curved field shapers face the three beams with their concave sides and make the first deflection field pincushion-shaped at the area of the electron beams.
A frequently occurring problem in colour display tubes having an electron gun system of the "in-line" type is coma. This is expressed in the fact that the dimensions of the rasters which are described on the display screen by the two outermost electron beams are different from those of the central beam. This is the result of the eccentric location of the outermost electron beams relative to the field for the vertical deflection (the frame deflection field). In U.S. Pat. No. 4,196,370 a large number of patents are cited in which partial solutions are given. These solutions consist of using magnetic field-conductive and/or screening rings and plates which are mounted at the gun end and which intensify or attenuate the deflection field or the deflection fields locally along a part of the paths of the electron beams. With a number of these means it is possible to cause the rasters described on the display screen by the three electron beams to coincide substantially. However, a disadvantage of the use of such means is that defocusing occurs in the outer beams during deflection which is expressed in a distorted spot on the display screen which is surrounded by a haze. One of the cited Patent Specifications is U.S. Pat. No. 3,594,600 which discloses a colour display tube in which the rasters described by the three electron beams are made to coincide by placing two elongate C-shaped magnetic screens beyond the outermost electron beams. As a result of this the outermost electron beams are screened from the edge field of the line deflection field (the vertical field lines), while the edge field is admitted to the central electron beam. The three electron beams are screened from the edge field of the frame deflection field (the horizontal field lines) which is bypassed entirely around the three beams. These field shapers thus influence only the line coma and not the frame coma.
Netherlands Patent Application No. 7801317, corresonding to U.S. Pat. No. 4,237,437, describes a system of deflection coils in which field-shaping means are provided in the system of deflection coils. They consist, for example, of two soft-magnetic elements which are provided diametrically opposite to each other beyond the line deflection coil, substantially transversely to the magnetic field of the frame deflection coil on the neck side of the system of deflection coils. The disadvantage of the use of such field-shaping means is that a considerable part of the field deflection field is bypassed around the beams by the means so that the deflection sensitivity of the tube-coil system decreases.
A colour display tube of the type described in the opening paragraph is described in Netherlands Patent Application No. 8204465, corresponding to U.S. patent application Ser. No. 851,282 filed on 10 Apr. 1986, which is a continuation of application Ser. No. 548,276 filed on 3 Nov. 1983 which may be considered to be incorporated herein. The field shapers described in these corresponding Patent Applications make the first deflection field (the frame deflection field) pincushion-shaped. This pincushion-shaped field comprises substantially a two-pole field having a six-pole component. As a result of the pincushion shape the field has the correct strength and shape also at the area of rays of the electron beams situated outside the electron beam axes, as a result of which the deflection focusing of the outer beams is considerably reduced. In contrast with the field shapers in the system of deflection coils according to Netherlands Patent Application No. 7801317, the field shapers disclosed in Netherlands Application No. 8204465 are situated comparatively closely to the electron beams, and therefore only a comparatively small part of the deflection field is distorted as a result of which only little extra deflection energy is necessary. It is also described in Netherlands Patent Application No. 8204465 that it is useful to provide slots in the field shapers and to manufacture field shapers from two or three plates situated in the elongation of each other. The object of this is to reduce the losses in the line deflection field (the second deflection field).
In Netherlands Patent Application No. 8301712, corresponding to U.S. patent application Ser. No. 607,544 filed on 7 May 1984, which may be considered to be incorporated herein, measures are taken to reduce the losses in the second deflection field. According to this Patent Application the field shapers each consist of at least two plates which are situated in the elongation of each other and which are situated in the manner as described in the opening paragraph relative to the plane through the beam axes and the tube axis. On the side remote from the electron beams the slots between the plates are overlapped at a distance from the plates by other plates, so that "magnetic shunts" for the second deflection field are created in each field shaper.